Almost without fail, in every single Simulation class I teach, someone asks me, “What’s the difference between Fixed and Immoveable?” They’re asking about two options in the Fixture dialog (often referred to as boundary conditions or restraints).
It occurred to me this week: Who better to explain fixins than a Thanksgiving Turkey! So, I fired up my SolidWorks 2012, and my turkey was done in about 30 minutes. Heh, it’ll take hours on Thursday!
Then, I got his mesh all set up. I used solid elements for his body, shell elements for his wings and tail, and beam elements for his legs. I had to make his legs a little longer to illustrate the point of this blog, but he’s still a handsome turkey I think.
Tech Tip: When using dissimilar mesh types, don’t forget to create Bonded Contact between them.
This is because the equations which define how an element responds to loading are different for each element type. The Bonded Contact tells the Mesh to create equations of compatibility so that the differing element types can pass information back and forth.
My turkey wings bonded automatically to their mounting posts. A few clicks later and I had the tail and legs properly bonded to the body. Mesh completed with the default element size.
Next, my turkey needs something happening to it. How about a 1 psi “tail wind”? Also, maybe another turkey charges his head with 22 pounds lateral force. For a little more color, let’s tug on those wingtips too with a little outward force.
Looks tasty!
For the first run, let’s select the beam joints at the turkey’s ankles and make them Fixed.
A few seconds of solving, and my turkey’s done. Boy does he looked stressed! I guess I would be too if I were a turkey at this time of year. [joke credit goes to Art Woodbury]
Tech Tip: Since beam element stress types are different than solid/shell stress types, you cannot display stresses of both element types at the same time.
I composited two plots together with an image editor to get that pretty picture for you.
Let’s look at a Displacement plot instead, since then I can show you the beams and the solids/shells all at once. Also the color gradient is more turkey-like. Matter of fact (Tech Tip!) you can customize the color legend to your liking (right click the plot and choose Chart Options > Color Options). I also changed the icon color of the fixtures from green to orange, and opted to show the non-deformed shape superimposed (right click the plot > Settings > Deformed Plot Options). Here’s some very turkey-like colors!
Now for the meat of this story [pun intended].
Let’s edit one of the fixtures on the turkey’s ankles and change it from Fixed to Immoveable. This means that the turkey’s ankle, instead of acting like it’s been stuck in concrete by the turkey mafia, it will be able to rotate. But it still can’t move off its spot. This is only true for shell elements and beam elements, because those elements calculate rotations at each node. Solid elements derive rotations by comparing the translation of nearby nodes, and so solids only calculate displacement. So…
Tech Tip: For solid elements only, there is no difference between Fixed and Immoveable fixtures.
Another few seconds in the solving oven, and our turkey looks redder, as his overall displacement increases. If you look at his left leg, you can see it is not as stiff a structure as before. The beam of his left leg rotates freely around the ankle joint, and therefore does not bend like his front leg does. You can also see that the icon representing the fixture is slightly different (arrows don’t have the disks on their ends, indicating that the rotation is not locked).
So the moral of the story is: If you have turkey with all the fixins, you’ll have less mobility than if you have turkey with just some fixins.
Next topic: Pumpkin Pi.
